Site Description A complex stone-walled fort occupying the summit of a volcanic outcrop overlooking the Gwynant valley. This is traditionally the site where Vortigern attempted to build a fortress and where the boy Merlin prophesied about dragons. Excavations, in 1910 and 1954-6, have produced evidence for Iron Age, Roman and early Medieval (Dark Age) occupation, whilst some of the surviving stone work is also thought to be medieval in date.

The fort extends over an area of about 200m east-west by 100m. The main, inner enclosure has an entrance on the west beyond which are two further lines of walls with gateways, the lowest close to the foot of the rock. The route up through the three gates is torturous, involving blind alleys and zigzags. The razor back ridge connecting the rock to the mountain proper offers a relatively straightforward approach.

At the highest point of the hill top are the remains of an oblong stone keep 13m by 11m, of medieval date. This is thought to be one of a small cluster of early stone-built castles constructed in Gwynedd at the end of the twelfth century, together with Castell Aber Ia (Castell Deudraeth) (NPRN 302700), Carn Fadrun (NPRN 95275), Tomen Castell (NPRN 303046) and Castell Pen-y-garn (NPRN 407747). These were not placed to withstand alien invasion, but were rather an expression of a Prince's power and lordship in the unsettled period following the death of Owain Gwynedd in 1170 and the subsequent division of the area between his sons.